Surface-activation of semiconductor nanostructures for biological applications

ABSTRACT

The present invention provides means and methods for producing surface-activated semiconductor nanoparticles suitable for in vitro and in vivo applications that can fluoresce in response to light excitation. Semiconductor nanostructures can be produced by generating a porous layer in semiconductor substrate comprising a network of nanostructures. Prior or subsequent to cleavage from the substrate, the nanostructures can be activated by an activation means such as exposing their surfaces to a plasma, oxidation or ion implantation. In some embodiments, the surface activation renders the nanostructures more hydrophilic, thereby facilitating functionalization of the nanoparticles for either in vitro or in vivo use.

FIELD OF THE INVENTION

This application relates to the production of semiconductornanoparticles, and in particular to the production of surface-activatedbiocompatible semiconductor nanoparticles.

BACKGROUND OF THE INVENTION

Semiconductor nanoparticles, or quantum dots, are nanometer ormicrometer-sized semiconductor structures in which one to a few thousandcharge carriers, e.g., electrons, are confined, giving them a uniqueability to emit visible or near infrared (IR) photons within a verynarrow spectrum and with high efficiency. Because of this, semiconductornanoparticles can be useful in numerous biological, genomic andproteomic applications, for example, as markers, as components ofmicrochip arrays (biochips), and as conjugates for fluoroimmunoassaysfor in vitro and in vivo molecular imaging studies.

At present, fluorophores synthesized from organic molecules typicallyare used in such applications. While organic fluorophores have had somesuccess, they tend to be unstable and gradually degrade when exposed toblue or UV excitation light, in a phenomenon known as photobleaching, orsimply degrade with time. Further, such molecules typically emit lightin the normal visible region, similar to that of other materials used inbiological applications, resulting in a poor signal to noise ratio.Organic fluorophores that emit light in the near-infrared region are notgenerally commercially available and, hence, difficult to obtain.

Inorganic semiconductor nanoparticle materials synthesized from CdSe andZnS-based materials are also used in biological, genomic and proteomicapplications. Because such inorganic materials can be toxic when used invivo, a coating of a protective material usually is necessary to renderthem useful in biological applications. However, as a result of thecoating, which absorbs and/or reflects light, the fluorescent propertiesof the material are diminished.

Moreover, many inorganic nanostructures are hydrophobic by nature andconsequently are not easily dissolved or suspended in aqueous solutions,rendering their use difficult in various applications, both in vitro andin vivo. Accordingly, there remains a need for better semiconductornanoparticles, especially for use in fluorescence-based biologicalapplications.

SUMMARY OF THE INVENTION

The present invention generally provides methods for producingsurface-activated semiconductor nanoparticles that exhibit colloidalstability and can be adapted for use in vitro and/or in vivo. Variousmethods are disclosed for activation of the surface of quantum dotstructures to render them more hydrophilic or to otherwise make themmore suitable for use with aqueous carriers and/or more easilyfunctionalized or conjugated with biological materials or reagents.Surface activation can be conducted via ion treatment, plasma treatmentor oxidation as an intermediate step in the production of the quantumdots or subsequent to initial harvesting of the semiconductor particles.

In one aspect, the present invention provides a method of producingsemiconductor nanostructures by generating a plurality of nanostructuresand activating their surfaces to render them more hydrophilic. Theactivation of the surfaces can be achieved, for example, by exposure ofthe nanostructures to ECR plasma, oxidation or ion implantation.

In another aspect, semiconductor nanoparticles can be formed by (1)generating a porous semiconductor layer on a substrate, said porouslayer including a plurality of porous nanostructures; (2) cleaving thenanostructures to generate a plurality of semiconductor nanoparticles;and (3) activating at least a portion of a surface of the nanoparticlesto enhance hydrophilicity thereof. The semiconductor can be an element,such as silicon, or a compound, such as GaAs. Porosity can be achievedby chemical or electrochemical etching. Cleavage can be induced, forexample, by sonication and, again, surface activation can occur beforeor after cleavage. Alternatively, the activation step can be performedbefore cleaving the nanostructures from the substrate.

In another aspect, semiconductor nanoparticles can be produced by (1)depositing a release layer on a semiconductor substrate; (2) forming aheterostructure over the release layer; (3) applying a plurality ofmasking nanoparticles to portions of the surface of the heterostructuresuch that part of the surface of the heterostructure can be selectivelyetched; (4) removing the exposed portion of the heterostructures whereina plurality of nanostructures are formed attached to the release layer;(5) activating at least a portion of a surface of the nano structures toenhance their hydrophilicity; and (6) dissolving the release layerwhereby semiconductor nanoparticles are formed. The nanostructures canbe activated by exposure to an ECR plasma, oxidation or ionimplantation, and the activation step can also occur after the releaselayer is dissolved and the semiconductor nanoparticles are formed.

The activated semiconductor nanoparticles are formed, they can befunctionalized with a biomaterial. For example, the biomaterial can be abiocompatible coating, a binding agent or a ligand. The functionalizedparticles can then be used for diagnostic, imaging and/or therapeuticpurposes. Alternatively, the activated nanoparticles can be disposed inan aqueous medium so as to generate a colloidal suspension.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating the various stepsperformed by one embodiment of a method according to the teachings ofthe present invention for creating surface-activated biocompatiblefluorescent semiconductor nanoparticles by formation on a poroussubstrate;

FIG. 2 is a schematic illustration of an etching system for generatingporous silicon substrates suitable for use with the present invention;

FIG. 3 is a schematic flow diagram illustrating the various stepsperformed by another embodiment of a method according to the teachingsof the present invention for creating surface-activated biocompatiblefluorescent semiconductor nanoparticles via epitaxial depositiontechniques; and

FIG. 4 illustrates a heterostructure formed on a wafer in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention provides methods for producingsurface-activated semiconductor nanoparticles suitable for in vitro andin vivo applications which can fluoresce in response to lightexcitation. The terms “nanoparticles,” “nanostructures,” and “quantumdots” are used interchangeably herein to describe materials havingdimensions of the order of one or a few nanometers to a few micrometers,more preferably from about 1 to about 100 nanometers. Preferably, theypossess an active device structure with optical properties derived fromthe excitation of a confined population of charge carriers.

The semiconductor nanostructures of the present invention can beactivated in various ways, such as by exposing their surfaces to aplasma, oxidation or ion bombardment. Once the surface is activated,functionalization of the nanoparticle for either in vitro or in vivo usecan occur.

So that the invention is more clearly defined, “activated” semiconductornanostructures or “activated” semiconductor nanoparticles arecompositions that have modified surface characteristics, e.g., morehydrophilic or hydrophobic, which facilitates functionalization orenhances colloidal stability.

In one embodiment, semiconductor nanostructures having activatedsurfaces can be generated by etching a surface of a semiconductor wafer,e.g., via an anodic etching process such as that described in moredetail below, to generate a surface layer comprising a nanostructuredporous network. The porous network can be subjected to an activationstep, e.g., via exposure to an ECR plasma or ion implantation, toactivate surfaces of the nanostructures forming the porous network.These activated nanostructures can be released as nanoparticles havingactivated surfaces by employing a variety of techniques, e.g.,sonication of the porous layer. Alternatively, prior to forming theporous layer, the semiconductor surface can be activated, e.g., via ionimplantation, followed by generating the porous layer and releasing thenanostructures.

By way of example, and as shown in a schematic flow diagram of FIG. 1,semiconductor nanoparticles can be formed by generating a porous siliconlayer over a semiconductor substrate. The substrate can be any suitablesemiconductor material known in the art, such as silicon, germanium,various compounds of Group III-V elements, or combinations thereof. In apreferred embodiment, the semiconductor material is preferably an n-type(phosphorus doped) or p-type (boron doped) single-crystal silicon wafer.Further, the single crystal silicon can have any suitable crystalinity,but is preferably has a (001) lattice orientation.

For example, in an initial step, a bulk substrate can be etched so as togenerate a nanostructured porous surface layer. FIG. 2 shows anexemplary etching system 1 for generating porous silicon substrates 10suitable for use with the present invention that includes a cathode 12,and an anode 14 disposed in a vessel 16, e.g., a Teflon vessel. Thevessel can contain a suitable electrolyte 18, for example, a mixture ofHF and ethanol, for anodically etching a surface of the siliconsubstrate, which is positioned in the holder such that one surfacethereof is exposed to the electrolyte solution while the other surfaceis isolated therefrom. A current source 20 generates a current flowingthrough the electrolyte solution to cause etching of the substrate'sexposed surface. The etching process can sometimes be enhanced if theexposed surface of the silicon substrate is first doped, e.g., dopedwith boron to render it a p-type semiconductor. The porous layer caninclude a network of nanostructures that can be activated and releasedas discussed in more detail below.

A person skilled in the art will appreciate that adjustments to theanodization conditions, such as the HF concentration, the pH of thesolution and its chemical composition, current density, temperature,etching time, stiffing conditions and illumination during the etchingprocess, can result in the porous silicon layer exhibiting variousproperties. For example, the use of an alcohol solution (e.g., a 1:1ethanol HF solution) may be preferable because it can reduce theformation of hydrogen bubbles at the substrate surface, therebyfacilitating production of a more uniform porous silicon layer.

Following formation, the semiconductor nanostructures can be cleavedfrom the substrate using any method known in the art to yieldfluorescent semiconductor nanoparticles. For example, in a preferredembodiment, the semiconductor nanostructures can be cleaved bysonication as taught in U.S. Pat. No. 6,585,947, herein incorporated byreference. Alternatively, the semiconductor nanostructures can becleaved to form semiconductor nanoparticles by shaking, scraping,pounding or any other technique whereby the semiconductor nanostructurescan be separated from the substrate. In some embodiment, followingcleavage, the semiconductor nanoparticles can be filtered using acommercial filter so as to remove any residual clusters and obtainnanoparticles having substantially uniform sizes in at a desired valueor in a desired range.

Referring again to the flow chart of FIG. 1, the surfaces of the cleavednanostructues can be activated to modulate their surface properties, forexample, to render them more hydrophilic. Although in this exemplaryembodiment, activation of the surfaces is performed following cleavageof the nanostructures from the porous layer, in other embodiments, thenanostructured can be activated while connected in the porous network,and then cleaved.

Such activation can result in semiconductor nanostructures that exhibitcolloidal stability and have increased hydrophobic or hydrophilicaffinities. Generally speaking, a “hydrophobic” compound is one thatlacks an affinity for polarized solutions, such as those containingwater; a “hydrophilic” compound is one that has an affinity forpolarized solutions, such as those containing water.

In one embodiment of the present invention, a plasma treatment techniqueis used to activate the surface of the semiconductor nanostructures. Byway of a non-limiting example, one type of plasma treatment techniquethat can be employed in the practice of the present invention iselectron cyclotron resonance (ECR), which allows a surface to bemodified via exposure to a spatially localized gaseous plasma.

In general, an ECR plasma can be generated by providing a staticmagnetic field having a selected strength, i.e., amplitude, within aregion of space in which a quantity of gas is contained, or throughwhich the gas is flowing. The gas is then irradiated withelectromagnetic radiation having a frequency which is substantiallyequal to ECR frequency at the applied magnetic field strength, andcauses the gas to ionize, thus producing a plasma. A surface to betreated is exposed to an ECR-generated plasma for a time period rangingfrom about one second to about one minute. While different exposuretimes can be selected for different modifications of the surface, forexample, shorter exposure times, such as one second, can be sufficientto activate the surface of the nanostructures.

Any combination of the radiation frequency and magnetic field amplitudethat substantially satisfy the following equation (Equation 1) can beused to obtain an ECR-generated plasma in accord with the teaching ofthe invention:

F _(c)=1/2π*(eB/m)  Equation (1)

where f_(c) denotes the ECR frequency, B denotes the amplitude of themagnetic field, and e and m denote the charge and the mass of anelectron, respectively. However, while various radiation frequencies andmagnetic field strengths can be utilized to create and ECR-generatedplasma, in a preferred embodiment, the radiation frequency can beselected to be in a range of about 1 GHz to about 15 GHz, and theapplied static magnetic field can be selected to have an amplitude in arange of approximately 300 Gauss to approximately 5500 Gauss. By way ofa non-limiting example, in one embodiment of the invention, thefrequency of the electromagnetic radiation can be about 2.45 GHz and theamplitude of the applied magnetic field can be approximately 875 Gauss.Alternatively, the frequency of the electromagnetic radiation can beabout 10 GHz when the amplitude of the applied magnetic field isapproximately 3571 Gauss.

Additionally, a variety of gasses and gas pressures can be used inconjunction with the magnetic field when forming an ECR plasma. Thesegases include, but are not limited to, noble gases, such as argon,diatomic gases, such as oxygen and nitrogen, hydrocarbons, such asmethane and butane, and fluorinated hydrocarbons, such astetrafluoromethane. Moreover, various mixtures of different gases can beutilized to create an ECR plasma in accordance with the teachings of theinvention. For example, a mixture of argon and oxygen (e.g., a mixturehaving 50% molar concentration of argon and 50% molar concentration ofoxygen) or a mixture of argon and ammonia can be used. Additionally, thegas pressure can be in a range of about 0.1 Pa to about 1000 Pa,preferably in the range of about 1 Pa to about 10 Pa, and mostpreferably about 2 Pa to about 8 Pa.

In one embodiment of the present invention, ion treatment can be used toactivate the surface of the semiconductor nanostructures. The term “iontreatment” and similar wording as used herein is intended to encompassion implantations, ion depositions, ion-beam-assisted deposition andion-enhanced sputtering. As used in the present invention, ion treatmentrefers to any treatment of a surface location (called a “localizedarea”) by utilizing energized ions. For example, an ion-beam-assisteddeposition (IBAD) process can be employed in which an ion source canaccelerate ions into selected portions of a substrate for implantationtherein. U.S. Pat. No. 5,520,664, herein incorporated by reference,provides further details regarding IBAD process and apparatus thereof.

The implanted ions can modify one or more surface properties of the nanostructures to modulate, e.g., enhance, their affinity forfunctionalization relative to an nanostructure suface not treated withions. Alternatively, activation of the semiconductor nanostructures canoccur by an ion implantation technique in which a selected number oflocalized areas are formed on a substrate surface by implanting one iontype in certain discrete regions of the substrate while other localizedareas are formed on the substrate surface by implanting another ion typein other discrete regions. This results in a substrate having two typesof localized areas, such that various localized areas have differentsurface properties relative to one another and/or relative to theremainder of the substrate surface.

In general, surface activation results in a desired modification of thenanoparticles' surface properties. One such modification can be a changein hydrophilicity or hydrophobicity. The term “hydrophilic” and itsderivatives are used herein to describe materials that have an affinityfor water and/or are capable of being dispersed in water. One measure ofa hydrophilic material is its ability to transfer from a non-aqueous toan aqueous phase in a dual phase system. For example, a “hydrophilic”compound typically will transfer from an organic phase to an aqueousphase, specifically from an organic, water-immiscible nonpolar solvent(e.g., with a dielectric constant less than 5) to water, with apartition coefficient or greater than about 50%. The term“water-dispersible” particles as used herein refers to an essentiallyunaggregated dispersion of particles, such that discrete particles ofapproximately 1 nm to 500 nm can be sustained indefinitely at highconcentrations (10-20 microMolar).

Any ion that is amenable to implantation in a given surface and thatleads to a desired surface modification can be utilized for surfaceactivation including, by way of a non-limiting example, nitrogen,oxygen, argon, carbon, fluorine, chlorine, hydrogen and helium. Further,the dose of the implanted ions at each localized area should besufficient to activate the area, and can be in a range of about 10¹² toabout 10¹⁷ ions/cm². More preferably, the dose can be in a range ofabout 10¹⁴ to about 10¹⁶ ions/cm². For example, a dose of nitrogen ionsin a range of about 10¹⁵ to about 10¹⁶ ions/cm² can be implanted atselected surface positions to provide a plurality of localized areaswhich are more hydrophilic than the remainder of the substrate surface.Alternatively, a dose of fluorine ions in a range of about 10¹² ions/cm²to about 5×10¹⁷ ions/cm² can be utilized to create localized areas thatare more hydrophobic than the remainder of the surface.

Alternatively, ion bombardment of selected positions on a substrate toimplant a selected ion at a specific position can be used to activatethe surface of the nanostructures. More particularly, in one suchmethod, initially, a mask is disposed over the substrate. The maskpermits selective treatment of the substrate surface, for example by anion beam. A variety of masks can be utilized to selectively exposedifferent portions of a substrate surface to the ion beam. For example,the mask can be formed of silicon dioxide (SiO₂). The mask can bedeposited on a silicon substrate, for example, by utilizing chemicalvapor deposition (CVD) to deposit a masking layer that can be patternedby employing a number of known methods, such as photolithography. Thepatterned mask can provide a plurality of exposed and unexposed portion.The substrate can then be exposed to a beam of ions, such as nitrogenions, having a selected energy based on a particular application (e.g.,an ion energy in a range of about 0.1 keV to about 1000 keV) so as toimplant a selected dose of ions in the exposed portions. A personskilled in the art will appreciate the variety of ion implantationsystems that can be employed for activating the surface of thesemiconductor nanostructures. The portion of the substrate implantedwith ions can be rendered porous, e.g., by employing an etchingtechnique such as that described above, to form a network ofnanostructures that can be released, e.g., via sonication, to generate aplurality of nanoparticles having activated surfaces due to the presenceof the implanted ions.

In another embodiment of the present invention, the surface of thesemiconductor nanostructures can be activated by oxidation. While avariety of oxidation techniques can be used, the oxidation techniqueshould preferably activate the surface of the semiconductornanostructures, while being controlled to such an extent that theoxidized semiconductor nanostructures would retain their fluorescentproperties. By way of a non-limiting example, the oxidation techniquecan include exposing the semiconductor nanostructures to an oxidizingatmosphere at an elevated temperature. While the oxidizing atmospherecan have a variety of compositions, in a preferred embodiment, itcontains at least about 1% O₂. Alternatively, the oxidizing techniquecan include immersing the semiconductor nanostructures in an oxidizingsolution for a length of time such that oxidation occurs. While theoxidizing solution can have a variety of compositions, a preferredoxidizing solution contains at least a percentage of sodium peroxide,nitric acid or sulfurous acid. Additionally, the length of the immersiontime can vary in accordance with the properties of the oxidizingsolution, for example and as shown by U.S. Pat. No. 6,649,138, hereinincorporated by reference, approximately 45 minutes to 1 hour is asuitable time frame for immersion in an H₂O₂ oxidizing solution so thatthe surface is oxidized at one monolayer thick.

While the semiconductor nanostructures formed by the above methodologiescan be cleaved to form semiconductor nanoparticles that have a varietyof average sizes, in a preferred embodiment, the average maximumdimension of the semiconductor nanoparticles is preferably between about0.5 nm to about 25 nm, and more preferably between about 2 nm to about10 nm. Further, while the absorption and emission maximum of thesemiconductor nanoparticles can vary, in one preferred embodiment theabsorption and emission maxima are between about 400 nm and about 1200nm, more preferably between about 500 nm and about 900 nm and, by way ofexample, between about 500 nm to 600 nm, about 600 nm to 700 nm, about700 nm to 800 nm or about 800 nm to 900 nm. In an exemplary embodiment,the semiconductor nanoparticles have a narrow emission spectrum (lessthan 75 nm, more preferably less than 50 nm) and a spectral separationbetween the absorption and emission spectra of more than about 20 nm,but preferably no more than about 50 nm.

One skilled in the art will appreciate that a variety of techniques canbe employed to form semiconductor nanostructures for use in accordancewith the present invention. For example, further techniques for formingsemiconductor heterostructures and then producing quantum dotnanostructures by etching and then releasing individual nanoparticlesare taught in co-pending commonly owned U.S. Patent Application entitled“Precision Synthesis of Quantum Dot Nanostructures for Fluorescent andOptoelectronic Device,” filed on even date herewith and incorporated byreference.

By way of example, FIG. 3 presents a schematic flow diagram depicting aprocess in accordance with another embodiment the present invention inwhich semiconductor epitaxial deposition techniques, together with theteachings of the invention, can be used to generated activatedsemiconductor nanoparticles. More specifically, semiconductornanoparticles are formed by depositing a release layer on asemiconductor wafer followed by deposition of additional layers thatform a heterostructure. A plurality of ion-blocking nanospheres are thendisposed on the surface of the heterostructure and the surface of theheterostructure is etched to remove the exposed portions, resulting inthe formation of semiconductor nanoparticles.

As shown in FIG. 4, a release layer 22 is deposited upon thesemiconductor wafer, which can be any suitable semiconductor so long asit is able to generate nanoparticles suitable for optical oroptoelectric applications. The semiconductor wafer preferablyincorporates Group III-V elements, such as GaAs, InGaAs or AlGaAs.Alternatively, the semiconductor wafer can be made of silicon orgermanium. Additionally, while the release layer may be made of avariety of materials, it should be of such material that can be evenlydeposited and easily dissolved, such as, other Group III-V elements, ordielectric materials deposited upon the substrate as pseudomorphiclayers. For example, in one embodiment, the release layer is formed ofAlAs. Those skilled in the art will appreciate that rather thanutilizing InGaAs and GaAs for forming the heterostructure layer, othersemiconductor materials, particularly other Group III-V elements, can beused. More generally, semiconductors useful in producing the quantumdots of the present invention can include II-VI, III-V and group IVsemiconductors. (Alternatively, using the new IUPAC system for numberingelement groups, suitable semiconductor materials include, but notlimited to, the following: materials comprised of a first elementselected from Group 2 or 12 of the Periodic Table of the Elements and asecond element selected from Group 16 (e.g., ZnS, ZnSe, ZnTe, CdS, CdSe,CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe,SrTe, BaS, BaSe, BaTe, and the like); materials comprised of a firstelement selected from Group 13 of the Periodic Table of the Elements anda second element selected from Group 15 (GaN, GaP, GaAs, GaSb, InN, InP,InAs, InSb, AlS, AlP, AlAs, AlSb, and the like); materials comprised ofa Group 14 element (Ge, Si, and the like); materials such as PbS, PbSeand the like; and alloys and mixtures (including ternary and quaternarymixtures) thereof.

The release layer can deposited upon the substrate by employing anydeposition technique known in the art, e.g., chemical vapor depositionor molecular beam epitaxy. Following application of the release layer tothe wafer, a heterostructure can be deposited on the release layer toprovide at least one one-dimensional confinement of electrons or othercharge carriers therein. As shown in FIG. 4, above the release layer 22,the heterostructure 24 includes a one-dimensional quantum well layer 26sandwiched between the two confinement layers 28, 30 that provideconfinement of selected charged carriers, such as electrons, within thequantum well layer 26. While the term “quantum well” is known in theart, to the extent that a definition may be needed, a “quantum well,” asused herein, refers to a generally planar semiconductor region, having aselected composition, that is sandwiched between semiconductor regionshaving a different composition (typically referred to as barrier orconfinement layers), selected to exhibit a larger bandgap energy thanthat of the composition of the quantum well layer 26. The spacingbetween the confinement layers 28, 30, and consequently the thickness ofthe quantum well layer 26, is selected such that charge carriers (e.g.,electrons) residing in the quantum well layer 26 exhibit quantum effectsin a direction perpendicular to the layer (e.g, they can becharacterized by discrete quantized energy levels).

Such heterostructures can have varied dimensions that can be selectedbased on desired properties of semiconductor nanostructures, such as, adesired emission spectrum. The thickness of various layers forming theheterostructure should be selected to allow an ion beam, utilized insubsequent etching steps, to sufficiently penetrate and etch awayselected portions of the heterostructure. Additionally, while theheterostructure can have a variety of heights, in an exemplaryembodiment, the heterostructure has a height ranging from about 1 nm toabout 10 nm, and preferably, a height of about 5 nm.

Referring back to FIG. 4, the quantum well layer 26 and the confinementlayers 28, 30 can be deposited on the wafer 20 by employing a variety oftechniques, such as organic chemical vapor deposition (MOCVD), and/orplasma assisted molecular beam epitaxial growth. It will be appreciatedthat MOCVD is particularly suitable for depositing Group III-Vsemiconductor materials on the wafer. By way of example, commonly-ownedU.S. Pat. No. 6,066,204, incorporated herein by reference, disclosesmethods and apparatus for epitaxial deposition techniques that can beutilized for generating the heterostructure layer 24.

In an exemplary embodiment, the one-dimensional quantum well layer 26 isformed of InGaAs while the confinement layers 28, 30 are formed of GaAs.The quantum well layer 26 can have a thickness in a range of about 1nanometer to about a few hundred nanometers, and more preferably, in arange of about 2 nm to about 20 nm while each of the confinement layers28, 30 can have a thickness in a range of about hundreds of nanometers.

Following formation of the heterostructure, and referring back to theschematic flow diagram of FIG. 3, a mask can be applied to a top surfaceof the heterostructure. For example, a plurality masking nanoparticlescan be applied to an upper surface of the heterostructure to form amask, e.g., a pattern of exposed and unexposed areas. In one embodiment,the mask protects part of the heterostructure from a subsequent etchingstep to which the wafer will be subjected, as described below.

While such mask can be formed by employing a variety of differenttechniques, in an exemplary embodiment the mask can be formed as acollection of gold or other metallic spheres having nanometer-sizedradii that can be deposited on the top surface of the heterostructure soas to protect selected portions of the heterstructure from subsequention beam bombardment, described below, while leaving the other portionsexposed. Additionally, the nanometer-sized mask particles can be formedfrom a variety of materials, however, such materials should inhibitoccurrence of adverse affects during etching, such as oxidation andpossible contamination of the wafer. Suitable mask materials for usewith the present invention include gold in a colloidal solution that canbe spread over a top surface of the heterostructure. Precision nanosizedgold spheres suitable for use in practicing the methods of the inventioncan be obtained, for example, from Accurate Chemical ScientificCorporation of Westbury, N.Y. in 1, 5, 10, 15 and 20 nm sizes.

Following application of the ion-blocking nanospheres to theheterostructure, the surface of the heterostructure is etched so that aplurality of nanostructure elements are formed, each including a portionof the quantum well heterostructure disposed on a portion of the releaselayer. While the nanostructures can be formed by any technique known inthe art, in a preferred embodiment, the nanostructures are formed byutilizing reactive ion etching or reactive ion beam etching.

Following irradiation, the release layer can be dissolved, if desired,to release the individual nanostructures forming semiconductornanoparticles. (If the mask particles have not been eroded away duringion etching, the mask reminants can be removed at this time also.) Whilethe release layer and the mask materials can be removed by a variety oftechniques, by way of a non-limiting example, the release layer and/orthe mask can be removed by dissolution in a suitable solvent, e.g., HF.

The surfaces of the released nanostructured can be activated byemploying the techniques described above, e.g., by immersion in anoxidizing solution. Alternatively, after deposition of the release layerand the heterostructure layer, the heterostructure layer can bebombarded with ions to implant a selected ion dose therein. Subsequentprocessing steps can be followed as described above to generatenanostructure with activated surfaces due to the presence of theimplanted ions.

Following activation of the nanostructures and cleavage to formsemiconductor nanoparticles or following activation of already-cleavedsemiconductor nanoparticles, the surface of the semiconductornanoparticles can then be functionalized with binding agent or ligand,e.g., a biomaterial that can form a biocompatible coating over theactivated surface.

The ligands can be attached to the activated surfaces via chemicalbonds, such as polar or non-polar covalent bonds, as well asnon-covalent bonds, ion bonds, metallic bonds, van der Waalsinteractions, or by cross-linking and caging.

In one embodiment, the surface-activated semiconductor nanoparticles canbe disposed in a medium, so as to form a long term suspension. Themedium can be any substance that can serve as a carrier for thesemiconductor nanoparticles and not affect their fluorescent properties.In one preferred embodiment, the medium is water based. Additionally,while the semiconductor nanoparticles can be dispersed throughout thesuspension in a variety of densities, in a preferred embodiment, thedensity of the nanoparticles can be in a range of about 1 microgram toabout 10 milligrams per milliliter.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not limited by what has been particularly shown anddescribed, except as indicated by the appended claims. All publicationsand references cited are expressly incorporated herein by reference intheir entirety.

What is claimed is: 1.-38. (canceled)
 39. A nanoparticle suspension,comprising: an aqueous medium, and a plurality of semiconductornanoparticles having dimensions in a range of about 1 nm to about 500 nmdispersed in said aqueous medium so as to form a suspension, whereinsaid semiconductor nanoparticles comprise activated surfaces forenhacing hydrophilicity thereof.
 40. The nanoparticle suspension ofclaim 39, wherein said activated surfaces comprise an oxide layer. 41.The nanoparticle suspension of claim 39, wherein said activated surfacescomprise a plurality of implanted ions at a concentration in a range ofabout 10¹² to 10¹⁷ ions/cm².
 42. The nanoparticle suspension of claim39, wherein said activated surfaces are generated by exposure of thesemiconductor nanoparticles to a plasma.
 43. The nanoparticle suspensionof claim 39, further comprising a plurality of ligands attached to saidactivated surfaces.
 44. The nanoparticle suspension of claim 39, whereinsaid ligands are attached to the activated surfaces via any of covalent,non-covalent, ion, and metallic bonds.
 45. The nanoparticle suspensionof claim 39, wherein said ligands are attached to the activated surfacesvia van der Waals interactions.
 46. The nanoparticle suspension of claim39, wherein said ligands are attached to the activated surfaces via anyof cross-linking and caging.
 47. The nanoparticle suspension of claim39, wherein a density of said nanoparticles in said suspension is in arange of about 1 microgram to about 10 milligrams per milliliter. 48.The nanoparticle suspension of claim 39, wherein a density of saidnanoparticles in the suspension is in a range of about 10 to 20microMolar.
 49. The nanoparticle suspension of claim 39, wherein saidnanoparticles comprise any of silicon, germanium, arsenic, Group II-VI,Group III-V semiconductors or a combination thereof.
 50. Thenanoparticle suspension of claim 39, wherein said nanoparticles includeany of p-type and n-type dopants.
 51. The nanoparticle suspension ofclaim 39, wherein the plurality of semiconductor nanoparticles have adimension in a range of about 5 nm to about 200 nm.
 52. The nanoparticlesuspension of claim 39, wherein the plurality of semiconductornanoparticles have a dimension in a range of about 2 nm to about 10 nm.53. The nanoparticle suspension of claim 39, wherein the plurality ofsemiconductor nanoparticles have a dimension in a range of about 0.5 nmto about 25 nm.
 54. A colloidal suspension, comprising: a medium, and aplurality of fluorescent semiconductor nanoparticles dispersed in saidmedium, said nanoparticles having activated surfaces for enhancingcolloidal stability and having a dimension in a range of about 1 nm toabout 500 nm, wherein said nanoparticles retain their fluorescentproperties in said medium.
 55. The colloidal suspension of claim 54,wherein said medium is water based and said activated surfaces enhancehydrophilicity of said nanoparticles.
 56. The colloidal suspension ofclaim 54, wherein said activated surfaces enhance hydrophobicity of saidnanoparticles.
 57. The colloidal suspension of claim 54, wherein saidactivated surfaces comprise any of an oxide layer and a plurality ofimplanted ions at a concentration in a range of about 10¹² to 10¹⁷ions/cm².
 58. The colloidal suspension of claim 54, wherein saidactivated surfaces are generated by exposure of the semiconductornanoparticles to a plasma.
 59. The colloidal suspension of claim 54,wherein a density of said nanoparticles in the colloidal suspension isin a range of about 1 microgram to about 10 milligrams per milliliter.60. The colloidal suspension of claim 54, wherein the plurality ofsemiconductor nanoparticles have a dimension in a range of about 5 nm toabout 200 nm.